How does the gregorian calendar differ from the Julian calendar?

The Julian calendar year was exactly 365.25 days.
The Gregorian calendar year is 365.2425 days.

While the difference is small (10.8 minutes), the effect was cumulative. Over the course of 1,200 years, the date of the vernal equinox had advanced by ten days. Since the Roman Catholic Church used the equinox to set the date of Easter, they considered it undesirable for it to be continually getting earlier in the year, so a change to the calendar was ordered by Pope Gregory XIII in 1582. The Julian Calendar is exactly 365.25 days long. Therefore, every fourth year, an extra day is added, called leap year. An actual solar year is 11 minutes less than 365.25 days long. The Julian Calendar gained three days every 400 years. The Gregorian Calendar was adopted in the 16th century which dropping some calendar days, in order to realign the calendar and the equinox times.